I may be rumdum from cleaning solutions (helped the wife clean the salon today) but I thought all our radiator caps were vented, which provides a safe, closed method of directing coolant overflow into an external reservoir and then, once the engine has cooled and everything has contracted, the return of the exhausted coolant back into the engine.
Essentially, it's a simple double gasket, double spring arrangment which provides a closed path both under pressure and under vacuum...
Or, am I missing something? *G*
The only thing I can think of is the old-style lever vent caps where you could manually vent the radiator pressure before removing the cap...
In the old days, there was simply a cap on the radiator (screw-on generally) which had a puke hole in it, hence the pictures of the old Model A's steaming away on the side of a long grade....when I would drive to parades, as a child, with a gentleman who's '13 Buick touring I helped maintain, I was the designated valvetrain oiler (exposed valvetrain) and radiator filler. I learned quickly about handling hot water and hot radiators safely....and keeping my fingers out of the valvetrain while the engine was running *G*